1. Field of the Invention
The present invention relates to a coordinate input device, a liquid crystal display device comprising the coordinate input device, and an electronic apparatus comprising the coordinate input device. Particularly, the present invention relates to a coordinate input device in which an operation surface can be scanned using an indicator to input coordinates, a liquid crystal display device comprising the coordinate input device, and an electronic apparatus comprising such a liquid crystal display device.
2. Description of the Related Art
Recently, as display units of electronic apparatuses such as personal digital assistants (PDA), a display unit has been known in which a transparent tablet (or a touch panel) is arranged on the front side of a liquid crystal display unit so that an operator can directly operate an object such as a menu item displayed on the display unit with his/her finger or an indicator.
FIG. 6 is a sectional view illustrating an example of a liquid crystal display device comprising such a tablet.
A liquid crystal display device 100 shown in FIG. 6 is constructed such that a tablet 130 is arranged on the front side of the liquid crystal display unit 120. In the liquid crystal display unit 120, a liquid crystal layer 123 is interposed between an upper substrate 121 and a lower substrate 122, which are arranged to face each other, and is sealed with a sealing material 124. A liquid crystal control layer 126 comprised of electrodes, alignment films and the like is formed on the inner side (the liquid crystal layer 123 side) of the upper substrate 121. On the inner side (liquid crystal layer 123 side) of the lower substrate 122, a reflecting layer 127 comprised of thin films made of a metal having high reflectance, such as aluminum or silver, and a liquid crystal control layer 128 comprised of electrodes, alignments films and the like are sequentially laminated on the lower substrate 122.
In the tablet 130, transparent upper and lower substrates 131 and 132, which are arranged to face each other), are integrally bonded to each other with a sealing material 136 (refer to, for example, Japanese Unexamined Patent Application Publication No. 10-104374). The upper substrate 131 is made of a flexible material, such as a transparent film, having a thickness of about 0.1 mm to about 0.2 mm. The lower substrate 132 comprises a plastic substrate having a thickness of about 0.5 mm to about 1.5 mm.
A first transparent electrode layer 133 is formed on the inner side (on the side of the upper substrate 131) of the lower substrate 132, and spacers (not shown) made of an insulating material are formed on the first transparent electrode layer 133 so as to be spaced from each other. A second transparent electrode layer 134 is formed on the inner side (on the side of the lower substrate 132) of the upper substrate 131.
A lower printed circuit (not shown) is formed at a peripheral portion of the lower substrate 132 on the inner side thereof, and the first transparent electrode layer 133 is connected to the lower printed circuit. An upper printed circuit (not shown) is formed at a peripheral portion of the upper substrate 131 on the inner side thereof, and the second transparent electrode layer 134 is connected to the upper printed circuit.
In such a tablet 130, since the upper substrate 131 is formed of a transparent film, the upper and lower printed circuits may be see-through. Thus, a frame 145 called a bezel is provided at the peripheral portion of the upper substrate 131 on the outer side thereof so that the printed circuits cannot be seen.
The tablet 130 having the above construction is of a resistive contact type. The upper substrate 131 is pressed with the indicator 140 so as to flex the first and second electrode layers 133 and 134, which are spaced from each other by the spacers when the tablet 130 is not operated. Then, the first and second electrode layers 133 and 134 are brought into electrical contact with each other so that coordinates can be input. Further, the tablet 130 is fixed to the liquid crystal display unit 120 via supporting members 139, and in the state in which the tablet 130 is fixed, the tablet 130 and the liquid crystal display unit 120 are spaced from each other with a predetermined gap therebetween.
According to the liquid crystal display device 100 of the above-described construction, display can be performed using the liquid crystal display unit 120, and an operation surface (the surface of the upper substrate 131) of the tablet 130 is scanned by the indicator 140 (a pen-type indicator in the drawing), so that coordinates can be input. Accordingly, when an operation is performed to select, for example, an object such as a menu item displayed on the liquid crystal display unit 120, the selecting operation can be performed by pressing a part of the display unit where the object is displayed, using the indicator 140.
However, in the liquid crystal display device 100 having the above construction, the upper substrate 131 of the tablet 130 is pressed and flexed by the indicator 140 at the time of input of coordinates. At this time, a pressing force is also applied to the liquid crystal display unit 120 arranged below the tablet 130, which may flex the upper substrate 121, changes a gap (cell gap) between the upper and lower substrates, and causes fluctuation in a display image. As a result, a problem may occur that it is hard for an operator to see the display image. Since the cell gap of the liquid crystal display unit 120 closely matches several micrometers, the cell gap of the liquid crystal display unit 120 changes locally in a large ratio although the upper substrate 121 is just slightly flexed. As a result, display may fluctuate and the visibility of display may deteriorate.
In addition, another example of the conventional tablet includes a tablet having the same structure as the tablet shown in FIG. 6 except that the lower substrate 132 comprising a plastic substrate is replaced with a lower substrate in which the underside of a transparent film (on the side of the liquid crystal display unit) having a thickness of about 0.05 mm to about 0.2 mm is adhered to the upside of a plastic substrate having a thickness of about 0.4 mm to about 2.0 mm. Even in the tablet of such structure, however, the same problem as in the above tablet may occur.
Accordingly, the spatial distance between the lower substrate of the tablet and the liquid crystal display device is generally set to about 0.3 mm to about 0.6 mm. However, the demand for tablets having a small flexure in the limited space increases.